Method for machining a workpiece and chuck with a center pin for implementing it

ABSTRACT

In order to machine a workpiece on a machine-tool successively between centres and with the workpiece clamped in a chuck, the workpiece is held during at least one machining operation—and if necessary between different kinds of machining—through the axial pressure of a centre pin clamped in a chuck of the machine-tool. To this end the chuck comprises an axial centre pin hydraulically movable in axial direction and which can be extended for enough towards the workpiece to be gripped by the jaws of the chuck.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority of European patentapplication No. 00 120 616.8, filed Sep. 21, 2000, the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] When machining a workpiece such as for instance a crankshaft withvery high precision it is sometimes desirable to perform this machining(turning, grinding etc.) in part between centers and in part with aworkpiece that is clamped in the jaws of the chuck of a machine-tool.This is because in order to machine parts of the workpiece that are faraway from its extremities and possibly eccentric, this workpiece must befirmly clamped at least at one of its extremities in the jaws of achuck, which means that this machining cannot be done between centers.Now, in order to obtain a precise centering of the workpiece when it isclamped in the chuck its jaws must size a surface that has a previouslybeen machined with a high precision with respect to a given rotationaxis. It is, however, difficult, or even impossible, to perform theprevious machining of this surface (which can for instance be the endbearing of a crankshaft) while corresponding extremity of the workpieceis clamped in a chuck, either because the surface to be machined wouldthen be so near to the chuck that it could not be reached by tools, orbecause if said surface remains readily accessible there is no placenear that end of the workpiece where the chuck could size it. Therefore,said end is often machined in a first, separate operation, betweenrotating center pins, which ensures a very precisely centered surface.This in turn guarantees that the workpiece will be precisely centeredfor later operations when it is seized by jaws that grip said previouslymachined surface, for example in order to perform an eccentric grindingof the workpiece.

[0003] The necessity to work on a workpiece that is sometimes maintainedbetween a center pins and sometimes in the jaws of chuck of themachine-tool has hitherto either required a resetting of themachine-tool or else the transfer of the workpiece from one machine tothe other. Both is uneconomical, time-consuming, and detrimental to theprecision of the final result.

BRIEF SUMMARY OF THE INVENTION

[0004] Hence, it is general object of dthe invention to avoid thesedrawbacks. In order to implement these and still further objects of theinvention, which will become more readily apparent as the descriptionproceeds, the invention is defined as recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The invention will be better understood and objects other thanthose set forth above will become apparent when consideration is givento the following deetailed description thereof. Such description makesreference to the annexed drawings, wherein the single FIGURE shows asschematical embodiment of a preferred device for performing the methodof the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0006] While there is shown and described a presently preferredembodiment of the inventnion, it is to be distinctly understood that theinvention is not limited thereto but may be otherwise variously embodiedand practised within the scope of the following claims.

[0007] The FIGURE shows a schematical section through two static statesof a chuck according to the invention intended for use in a machinetool. Cipher 1 identifies the body of the chuck, A its rotational axisand 2 a jaw of the chuck. A center pin 3 is a journalled in the chuck inaway which allows its displacement parallely to the axis A. In the upperhalf of the FIGURE the center pin is shown in its forward position, andin the lower part of the FIGURE it is shown in its retracted position.Accordingly, in the upper section of the FIGURE a workpiece 4 located onthe right hand side is supported and centered through a center pin 3 atits extremity facing the chuck, whilst in the lower part of the FIGUREit is clamped in the collet of the chuck (of which only jaw 2 is visiblein the drawing). The workpiece is then not supported by the center pinanymore, although it may still touch it.

[0008] The upper half of the FIGURE shows the machining step usuallyperformed first, when a surface S situated near one end of the workpiece4 is being machined (for instance ground). To perform this operation thecenter pin 3 is moved outwards, for instance through the pressure of ahydraulic fluid that is pressed through a conduit 6 in the main body ofthe chuck into a chamber 8. Once the center pin has been extendedoutwards it is seized along the surface 10 indicated by a thick line bythe jaws of the chuck which move in the usual way, i.e. radially in thedirection of the double arrow R.

[0009] The center pin 3 is thereby centered with a precision which ispractically equal to that of the jaws 2 of the chuck, independently ofthe play of the center pin in the journal which maintains it in a waythat allows an axial movement within the body 1 of the chuck. When anextreme precision and/or an extremely long tool life is required thecenter pin 3 can be journalled in two so-called expanding sleeves 5, 5′which can be expanded in order to additionally maintain the center pin.Such expanding sleeves are known in the trade and comprise an annularchamber with a thin cylindrical inner wall the radially inward surfaceof which forms a friction bearing for the center pin. When this chamberis filled (through a conduit that is not shown) with a pressurized fluidits thin inner wall deforms slightly radially inward and seizes thecentering pin, i.e. it suppresses the play necessary for the axialmovement of the center pin. The center pin 3 can thus be fixed in aprecisely centered position and be freed again with the least possibleplay.

[0010] When the center pin is extended it maintains the workpiece sothat the surface S near its left-hand end (which can for instance be theend bearing of a crankshaft) sufficiently faraway from the chuck and itsjaws to be easily accessed by the machining tools. In this position thesurface S can therefore be machined with the high precision renderedpossible by a machining between centers.

[0011] The necessary rotation of the workpiece can for instance beobtained by a driving dog 12, represented in the upper section of thedrawing in the form of a broken pin, which transmits (in a way notshown) the rotation of the chuck from one jaw of the same to theworkpiece. Machining between centers can, however, also be performedwith an immobile chuck and pin, the rotation of the workpiece being thenobtained in a known way (not represented) i.e. through a transmission bybelt or some other means.

[0012] Thereafter the jaw 2 is released and the center pin retracted,which can be achieved in different ways for instance through thepressure of a hydraulic fluid pressed through a conduit 7 into a chamber9, but also through the axial pressure directed towards the left andindicated in the FIGURE by the arrow D exerted by the workpiece 4against the pressure of this fluid and. One can then safeguard theseating of the workpiece 4 on the center pin 3 in the following wayduring its displacement: the force which pushed the center pin towardsthe right during the previous machining operation (here the hydraulicpressure in the chamber 8) is not suppressed entirely, but only reducedin a way which at all instants ensures a reliable support of theworkpiece 4 through the center pin. One then needs no additional meansfor supporting the workpiece during the transition from one machiningoperation to the next. When machining on a twin-machine-tool, where theother extremity (not shown) of the workpiece is supported in anessentially similar, but symmetrical way by a second head of themachine-tool it is for instance possible to synchronize the movements ofboth chucks and center pins in a way which ensures that the mountedworkpiece is reliably maintained through the pressure of the center pinsand/or the grip of the chucks of both machine heads during the passagefrom one machining operation to the next.

[0013] As represented in the lower half of the FIGURE, once the centerpin 3 has been retracted into the depths of the chuck 1 the workpiececan be displaced sufficiently far to the left to ensure that is surfaceS which has been previously machined between centers lies between thejaws 2 of the chuck. The jaws are then closed in the usual way in orderto seize the surface S at the location 11 indicated by a thick line,thus adequately maintaining the workpiece 4 for machining operationseither on a more distant surface T, or on some excentric surfacesituated further away (not shown).

[0014] Contrary to a support between centers such a clamping in a chuckcan be sufficiently robust and maintain a sufficiently precise angularorientation for machining eccentric parts of the workpiece, such asconnecting rods bearings on a crankshaft.

[0015] In the position shown in the lower part of the FIGURE thepressure of the center pin 3 towards the right on the workpiece 4 isentirely suppressed, as evidence by the small gap between both parts.However, this pressure may also be simply maintained low enough to beirrelevant for the positioning of the workpiece; in both cases theworkpiece is centered exclusively through the action of jaw 2 and theother jaws (not shown) of the chuck. Therefore, when retracted, thecenter pin 3 needs not rotate in a way that is centered with absoluteprecision, which noticeably reduces the complexity of its bearings andcan make the use of the above-mentioned expanding sleeves superfluous.

[0016] It is note worthy that although one and the same part of the jawscan be used both for seizing the center pin and the workpiece, thepreferred embodiment described here uses different parts of the jaws forthese two purposes, to wit: part 10 of the jaws serves to clamp thecenter pin 3 and part 11 of the jaws serves to clamp the workpiece 4.This separation allows an optimal adaptation of the diameter, andpossibly of the shape and/or the type of the corresponding surfaces, ofthe jaws to the intended purpose. This freedom of choice in particularallows to avoid damaging the surface S, even when one must transmit acomparatively large torque of for instance 100 N·m, such as is forinstance needed for grinding.

[0017] While there are shown and described presently preferredembodiments of the invention, it is to be distinctly understood that theinvention is not limited thereto but may be otherwise variously embodiedand practiced within the scope of the following claims.

1. Method for machining a workpiece in a machine-tool comprising atleast one tailstock, characterized in that one uses a center pinjournalled in an axially displaceable way in a chuck of the machine-toolfor maintaining the workpiece at its extremity facing the correspondingheadstock during at least one machining step.
 2. Method according toclaim 1 for machining a workpiece in a twin-head machine-tool comprisingtwo mutually opposite headstocks, characterized in that each headstockis provided with a center pin journalled in an axially displaceable wayand that one uses during at least one machining step a center pinclamped in the chuck of a headstock for maintaining the workpiececentered at its extremity which is neat that headstock.
 3. Methodaccording to claim 1 or 2, characterized in that during that during onemachining step the rotation of a chuck is transmitted to the workpiecethrough a dog revolving with the chuck.
 4. Method according to claim 1or 2, characterized in that during that during one machining step theworkpiece is rotated by means independent of the rotation of the chuck.5. Method according to any preceding claim, characterized in that duringat least one other machining step a center pin is free from the jaws ofthe corresponding chuck and retracted behind said jaws, which then holdthe workpiece.
 6. Method according to claim 4, characterized in that atleast one chuck is at rest during said one machining step.
 7. Methodaccording to claim 6, characterized in that the section of the jaws ofat least one chuck which hold the center pin during said one machiningstep differs from the section which holds the workpiece during saidother machining step.
 8. Method according to any preceding claim,characterized in that said one machining step comprises the machining ofa part of the workpiece that is nearer to the corresponding headstockthan the part being machined during said other machining step.
 9. Methodaccording to any preceding claim, characterized in that one step isperformed first and another later, and that during the later machiningstep the jaws seize the workpiece on a surface machined during the firstmachining step.
 10. Method according to any of claims 6 to 9,characterized in that during at least part of the time between said oneand said other machining step the workpiece is maintained at least atone of its extremities through a center pin which presses against thisextremity.
 11. Method according to any preceding claim, characterized inthat the at least one center pin is urged hydraulically or pneumaticallyalong the rotation axis of the chuck.
 12. Chuck for performing themethod according to any of claims 1 to 11, characterized in that itcomprises at least one center pin journalled centrally in the chuck anddisplaceable along the its rotation axis so that in its foremostposition the center pin lies within the gripping space of the chuck andthat in its rearmost position the center pin lies outside said space.13. Chuck according to claim 12, characterized in that the center pin isdisplaceable by automatic means.
 14. Chuck according to one of claims 12or 13, characterized in that the center pin is journalled in at leastone expanding sleeve bearing.
 15. Chuck according to any of claims 12-14for performing the method according to any of claims 6-11, characterizedin that within a first axial interval its jaws are shaped for seizingthe center pin and that within a second such axial interval, differingfrom the first, its jaws are shaped for seizing the workpiece.